A typical insulated gate device (IGD) is the insulated gate transistor (IGT), a device capable of controlling a high forward current with a low bias voltage applied to its gate electrode. This gate control characteristic makes the IGT particularly useful for power control and current switching applications.
A typical IGT is implemented on a silicon semiconductor wafer. This wafer includes a distributed drift layer of a first conductivity type overlain by a major wafer surface. This drift layer is underlain by an adjoining, distributed collector region of a second conductivity type. A plurality of cells are situated in the wafer for conducting a main device current, each cell comprising a base region of a second conductivity type extending from the major surface into the drift region, and an emitter region of the first conductivity type extending from the major surface into the base region. Each cell emitter region is spaced from the distributed drift region so as to form a channel in an intervening portion of the cell base region.
A distributed gate electrode is insulatingly spaced from the major surface of the wafer and overlies each cell channel, such that a bias voltage applied to the gate electrode causes an electrical field to be developed for controlling the magnitude of a main device current flowing through the channel. Distributed emitter and collector electrodes are connected, respectively, at opposite ends of each cell, and conduct the main device current into and out of the cells. A typical collector electrode comprises a conductive layer disposed in continuous ohmic contact with the distributed collector region. A typical emitter electrode comprises a conductive layer disposed in ohmic contact with emitter and base region surfaces of each cell exposed at the major surface of the wafer.
As is known to those skilled in the art, the emitter, base and drift regions of the typical IGT cell form a first, inherent bipolar transistor, while the base, drift and collector regions form a second, inherent bipolar transistor. These first and second inherent bipolar transistors have respective forward a current gains of .alpha.1 and .alpha.2, and by the nature of their construction are regeneratively coupled to form a parasitic thyristor. This paraisitic thyristor is susceptible to latching when the sum of the forward current gains of the two bipolar transistors equals or exceeds unity. When this parasitic thyristor latches, the IGT loses gate control of the forward current, and can only be turned off through an external action such as commutation.
As is further known to those skilled in the art, one cause of parasitic thyristor latching is a reverse current flow of minority carriers through the base region of a cell adjacent the emitter-base junction of the first bipolar transistor. This reverse current, whose magnitude increases as the main device (forward) current increases, causes a voltage drop to develop along the emitter-base junction which, when it exceeds a threshold voltage, forward biases the junction and causes the forward current gain of the first bipolar transistor to substantially increase. Hence the sum of the forward current gains of the first and second inherent bipolar transistors has a high probability of exceeding unit and causing the parasitic thyristor to latch. Because the active cells are parallel connected, when the parasitic thyristor in any one cell latches, the remainder also latch and the IGT loses gate control of the forward current.
It has been discovered by the present inventor that, due to certain structural characteristics, a typical IGT contains "hot spots", or sites where there is a disproportionately high density of reverse current in comparison to the remainder of the device. Cells situated in the vicinity of these hot spots experience an unusually high density of reverse current flow adjacent their emitter-base junctions and, for the reasons described above, have more of a tendency to go into a latched, uncontrollable state of operation than do the other cells in the IGT.
Specifically, these hot spots have been discovered at three sites in a typical IGT. The first site is that part of the IGT underlying a metal gate electrode spine. As is known to those skilled in the art, a continuous, metallic, gate electrode spine overlays selected portions of the gate electrode and is ohmically connected thereto at a plurality of discrete locations. This spine has the effect of diminishing the resistive (R), inductive (L) and capacitive (C) transmission line characteristics of the distributed gate electrode structure. In typical IGT constructions, accommodating the spine requires that the separation be increased between those active cells located along opposite sides of the gate electrode portions overlain by this spine. Thus, there is created a larger volume of drift region situated under the gate electrode spine. Unfortunately this larger drift region volume supports an increased minority carrier concentration which produces the disproportionately high reverse current density in the cells located proximate the gate electrode spine.
A second hot spot on the typical IGT is situated at the site adjacent the metal gate electrode pad. This metal gate electrode pad comprises a portion of the distributed gate electrode structure to which electrical connection of an external lead wire is conveniently made. A blocking region of the same conductivity type as the base region underlies the gate electrode pad and is in turn underlain by a portion of the drift region, thereby providing a three layer construction for preventing main device current from flowing to the gate electrode pad. A large volume of drift region also exists proximate the gate electrode pad and supports a high concentration of minority carriers capable of producing a disproportionately high reverse current in the manner described above. Hence, those IGT cells located proximate the blocking region underlying the gate electrode pad also experience a high reverse current density flowing adjacent their emitter-base junctions, and thus exhibit the tendency to latch before the remaining cells in the IGT.
The third hot spot is the site on the typical IGT adjacent a metal emitter electrode pad. This emitter electrode pad, in a manner similar to the gate electrode pad, comprises a section of the emitter electrode to which connection of an external lead wire may be conveniently made. This emitter electrode pad is underlain by the same type of three layer structure as the metal gate electrode pad, but, in contrast to the metal gate electrode pad, the emitter electrode pad is ohmically connected to its underlying blocking region at a limited number of selected locations. These ohmic connections between the emitter electrode pad and the underlying blocking region route minority carriers from a portion of the drift region situated proximate the emitter electrode pad to the emitter electrode pad along paths well spaced from any neighboring cell's emitter-base junction. It has been discovered, however, that the volume of drift region situated proximate the emitter electrode pad is so large that it supports a concentration of minority carriers too high to be safely drained through the paths established by these limited ohmic contacts. Thus, a high reverse current density exists here as well, with the potential to latch those IGT cell proximate the blocking region underlying the emitter electrode pad.